Conventional domestic hot water storage systems typically combine solar heating and back up heating. One common solar water heating system architecture is known as the thermosyphon system, shown in FIG. 1, a diagram of a conventional thermosyphon system. The thermosyphon system, although cost effective and usable in temperate and warm climates, tends to suffer from clogging due to scale and corrosion. This system is energetically inefficient in cold climates due to the need to heat the outdoor tank to the required domestic hot water (DHW) temperature (65 degrees Celsius [° C.] in many cases in order to prevent risk of Legionnaire's disease) and heat losses due to reverse thermo siphoning during colder periods, such as at night.
Refer to FIG. 2, a diagram of a conventional combi-system, an alternative to the thermosyphon system and more common in colder climates. The combi-system separates the tank 200 from the collector 202 allowing the tank 200 to be placed inside (a dwelling). This configuration somewhat reduces heat losses in cold climates but still requires the heating of the full tank as in the above-described thermosyphon system. Since the external collector loop 204 is exposed to potentially freezing temperatures, this loop is generally filled with glycol. In this case, regulations in many European countries require an additional heat exchanger stage (not shown) to prevent risk of contamination of potable water. This regulation further reduces energy efficiency.
Refer to FIG. 3, a diagram of an alternative conventional combi-system. This alternative combi-system is an improvement on the above-described combi-system (of FIG. 2). This improved system overcomes the regulatory requirement with the tank-in-a-tank architecture but substantially increases cost.
Most of the above conventional systems also suffer from relative slow response time and fluctuating temperatures when the consumer opens the hot water tap, unless an additional flow generator and flow loop is included in the hot water system, further increasing system cost. A further risk factor for solar water heating systems is that of pipe and collector freezing. A common solution to this problem is the use of glycol or other anti-freeze agents in the solar heat transfer loop. This common solution is costly, requires periodic replacement of the glycol, a drain-back system, and an overheating prevention device to safeguard against damage to the glycol as well as regulatory system complications as specified above.
There is therefore a need for an improved system for solar assisted water heating, having lower cost, higher energy efficiency, and quicker response time than conventional solutions.